Synthesis of Nanoplatelet Zinc Borate and its Combination with Expandable Graphite and Red Phosphorus as Flame Retardants for Polypropylene

The zinc borate nanoparticles (2ZnO.3B2O3.3H2O) were successfully prepared by precipitation reaction in aqueous solutions of borax and zinc sulfate using oleic acid as a surfactant. The structural, morphological characteristics and the wettability of the particle surface were studied through Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the contact angle. The hydrophobic zinc borate (n-ZB) had nanoplatelet morphology with a diameter of 1.0 - 1.5 µm and thickness of about 90 nm. Nano zinc borate showed a synergistic influence with expandable graphite (EG) and red phosphorus (RP) on the flame retardant properties and thermo-oxidative stability of polypropylene (PP). The nanocomposite 7n-ZB/7RP/7EG/PP achieved the V-1 UL94 verticle burning test with a limited oxygen index of 23.7% and the char yield of 14.67 wt.% at 900 oC. Furthermore, the fire retardant performance and the mechanical properties of the nanocomposites loading ZB nanoparticles were improved compared with the composites employing commercial ZB microparticles.